Effect of nematode-trapping fungi on an entomopathogenic nematode originating from the same field site in California

We determined whether nematode-trapping fungi may influence the dynamics of a coastal shrub community. The food chain interactions in the shrub community involve the dominant plant species, its major insect herbivore, and an entomopathogenic nematode, Heterorhabditis hepialus. Of the 12 nematode-trapping fungi previously isolated from soils at the study site, 5 were selected for this study. Arthrobotrys oligospora, Geniculifera paucispora, Monacrosporium eudermatum, and Monacrosporium cionopagum efficiently trapped and colonized H. hepialus on agar; in contrast Nematoctonus concurrens trapped but did not infect or colonize the nematode on agar. To determine whether these fungi can suppress N. hepialus in soil, we added the fungi in the form of fungal-colonized nematodes to pasteurized (2 hr at 62 degrees C) and raw (nontreated) soil from the study site. Suppression was measured by comparing nematode invasion into a wax moth larva in fungus-treated and untreated soil in vials at 20 degrees C. Fungal population density in soil was estimated using dilution plating and most probable number procedures. All fungi suppressed H. hepialus if the wax moth larvae were added 4 days after the nematodes. Suppression ranged between 37 and 54% and did not differ among fungi. Suppression was usually greater in raw than in pasteurized soil. Raw soil contained a constant background of nematode-trapping fungi, and A. oligospora was the most common among these; no background was detected in pasteurized soil. The presence of background fungi in raw soil may explain the higher suppression in raw than in pasteurized soil. Fungal propagule densities in our laboratory experiments were similar to those observed in the field, suggesting that nematode-trapping fungi may influence the dynamics of the plant, insect herbivore, and entomopathogenic nematode in the coastal ecosystem.